Microwave heating apparatus

ABSTRACT

A microwave heating apparatus is disclosed, the apparatus being provided with a treatment chamber, a microwave source, and a wave guide having a mode converter. A first section of the wave guide connected to the treatment chamber is joined to a second section connected to the microwave source and has a rectangular crosssectional area with sides approximately equal to a and b under formation of a so-called H-bend, the first section having a cross-sectional area in the form of a rectangle with sides approximately equal to n X a and b, where n 2, 3, 4, etc., and wherein the H-bend between two outer sides of the respective wave guide sections have a corner-plate parallel to the rectangle side b of the wave guide cross-section areas and form inside obtuse angles with the outer side. The apparatus ensures spreading and distribution of the radiation in the treatment chamber with an even intensity.

United State sPatent 1 19 1 Ris man et al.

[451 May 7, 1974 MICROWAVE HEATING APPARATUS Primary Examiner-Joseph V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney, Agent, or Firm-Holman & Stern ABSTRACT A microwave heating apparatus is disclosed, the apparatus being provided with a treatment chamber, a microwave source, and a wave guide having a mode converter. A first section of the wave guide connected to the treatment chamber is joined to a second section connected to the microwave source and has a rectangular cross-sectional area with sides approximately equal to a and b under formation of a so-called H- bend, the first section having a cross-sectional area in the form of a rectangle with sides approximately equal to n X a and b, where n 2, 3, 4, etc., and wherein the l-l-bend between two outer sides of the respective wave guide sections have a corner-plate parallel to the rectangle sideb of'the wave guide cross-section areas and form inside obtuse angles with the outer side. The apparatus ensures spreading and distribution of the radiation in the treatment chamber with an even intensity. v

7 Claims, 5 Drawing Figures [75] Inventors: PerOlov G. Risman, Vastra, Sweden; Lars O. Rosengren,

Washington, DC. [73] Assignee: Husgvarna Vapenfabriks Aktiebolag, Huskvarna, Sweden [22] Filed: Oct. 12, 1971 21 Appl. No.: 188,015

[30] Foreign Application Priority Data 00!. 19, 1970 Sweden l4061/70 [52] US. Cl. 219/1055, 333/98 BE [51] Int. Cl. 05b 9/06 1 [58] Field of Search..... 219/1055; 333/21 R, 98 BE [56] References Cited V UNITED STATES PATENTS 3,539,75'l ll/l970 Levinson 2l9/l0.55 2,814,708 1 H1957 Blass 2l9/l0.55 2,673,962 3/l954 Kock 333/98 BE 3,662,141 5/l972 Schauer.... 2l9/l0.55 2,600,566 6/1952 Moffett;...j. 2l9/l0.55 UX 2,909,635 10/1957 Haagensen 2l9/l 0.55

sum 2 or 2 PATENTEDMAY 1 m4 w a m a qd 18 FIGLI' FIG. 5'.

. 1 MICROWAVE IEATING APPARATUS BACKGROUND OF THE INVENTION The present invention relates to a microwave heating apparatus in which microwave energy is conducted from a microwave source to a microwave heating applicator by means of a waveguide, provided with a mode converter.

In microwave heating appliances the need arises-frequently for an extended heating zone in a treatment chamber, in which the article to be heated passes the zone at a specified, desirable speed. The problem arises as to how, from a relatively narrow cross sectional area in, for example, a waveguide, a spreading of the radiation with an even intensity within a large area, determined by the type of the furnace, could be obtained.

PRIoR ART it provides in a narrowoven, the greatest density of radia'tion in the center of the heating zone, since its width corresponds to approximately half the wave length of the microwaves. The result is an uneven heating of the article and, as a consequence thereof, a reduction of the maximum applicable effect. Since the greatest advantage of the microwave heating consists in the short treatment times, there is every reason to attempt to overcome the disadvantage of uneven radiation owing to the TE wave shape by the application of some other wave shape an combination of wave shapes SUMMARY OF THE INVENTION The problem is solved in accordance with the invention in that a more uniform field of radiation is achieved by means of the conversion of at least a portion of the TB wave issuing from the microwave source to waves of a higher order, for example TE By building up the radiation energy of several different wave shapes, the intensity at the outer edges of the treatment chamberbecomes appreciably higher than at radiation with only the TE wave. As a result, the treatment zones become wider and the capacity of the oven is increased. For the conversion, a converter specially designed for the purpose is used, which is very simple and effective. Its principal characteristics for fulfilling the function which leadto the improvement of the microwave oven are defined in such a way that a first section of a wave guide connected to the treatment chamber is joined to a second section connected to the microwave source and having a rectangular cross-section with sides approximativelya and b, under formation of a so-called H-bend, said first section having a crosss'ection in the form of a rectangle with sidesa'pproximatively n a and b where n 2, 3, 4cm, and that said I-I-bend between two outer sides of the respective waveguide sections has a corner-plate parallel to the rectangle side b of the waveguide cross-sections and forming inside obtuse angles with the outer sides. The

' present converter constitutes a substantial part of the microwave system used in the apparatus according to the invention and enables a quite unique type'of applicator.

An example of the realization of a microwave heating appliance provided with a waveguide and modeconverter and an applicator connected thereto, all in accordance with the invention, is described in the following with reference to the enclosed drawings, in

which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing such an appliance in a combination oven-hotplate or surface unit,

FIG. 2 is a detailed view of a waveguide switch used in the appliance according to FIG. 1,

FIG. 3 is a cross-section of a vessel with a base, constituting an absorber, intended to be used on a cooking plate according to FIG. 1,

FIG. 4 shows in a horizontal projection a waveguide with a mode-converter and FIG. 5 shows a vertical projection of the waveguide.

DETAILED DESCRIPTIONVOF THE INVENTION,

The appliance shown in FIG. l'can be used for the warming up of an article in an oven 1 as well as for cooking and roasting in pot or like receptacle on a plate or surface unit 2. The oven has atight-closing door 3 on the frontside and is suitable for being placed on a kitchen range or for being built into a cupboard. The cookingplate is fixedto one side of the oven and has a panel equipped with control knobs 4 and indicator lamps 5. The plate and the oven each form the base for a microwave applicator or radiation surface 6 and re spectively. This is defined as a device which, without being heated itself by the microwaves incident on it from generators, conducts and modifies the field pattern, so that the same is made suitable for absorption by the object or objects which are to be treated by microwaves. A waveguide system conducts the radiation from a magnetron 8 to the respective applicator. The magnetron has an aerial 9 which is located in the end of a main waveguide 10 which is provided with a waveguide switch 11 from which issues a branch waveguide 12. The waveguide switch, which is shown :in FIG. 2,

has a rotary handle 13 which is accessible from the outside and which is used to change over a pivotable plate 14 between a position oblique to the branch conductor and a position parallel with the main conductor. In the first named position, the radiation from the magnetron is reflected into the branch conductor and in the second position the radiation continues in the mainguide. Behind the oven chamber is an apparatus chamber The two waveguides are each by itself provided with mode converters. Such a converter consists for example of a waveguide with a waveguide passage in form of a corner, for example as formed by the main waveguide 10 and a waveguide tube 15 at an angle to the same. The main waveguide according to FIG. 1 is designed as a TE -waveguide and conducts the TE waves emitted by the magnetron to the waveguide corner. This comprises the connection of the tube 15 at an angle with the main guide 10 as well as a corner plate 16 between two outer sides 17, 18 of the respective tubes. The cross-section of the tube 15 defines a rectangle with a long side of approximatelydouble the length of the corobtuse angle'therewith. The plate reflects the TB waves incident from the main guide into the tube with simultaneous conversion of a part of the TE waves to TE waves. The combined radiation of TE and 'TE type, which is obtained in the normal manner by squaring and halving of the amplitude values, is thus sectional area in different parts, than is the case in a pure TE Waveguide.

The converter shown in FIG. 4 constitutes a simple example of how a wave type can be converted, while the principle also comprises multiple and complex con- .vers'ions of thistyp'e. lf, for example, several of the waveguide passages now described arecoupled behind one another, or if a specially wide tube replaces the tube 15, it can be stated in advance that the radiation issuing will include wave shapes TE TE TE wherein, however, certain terms may dominate while others will be wholly excluded. 7

The construction of the converter shown is very simple to realize, since the sides of the waveguides are made up of edge-flanged sheetsand even the corner plate 16 can be made of such sheet, which means that the converter is built only of elements generally used for waveguides. It is also possible to arrange in the waveguide tube a control element for the propagation of the waves; thus two parallel slots 15a are present in the tube 15 on both sides about the central plane. In these slots which are in the proximity of the amplitudes of the TE' -wave so-calledchokes could be arranged,

the size and position of which are such that the characteristic of the waveguide for TE waves is improved.

The tube 15 has on the topside the radiation surface 6 surrounded by a ring or frame 19 and provided with a number of slots 20 in the wall of the tube. As the end ,of tube 21 is closed by a plate 22, the microwave radiation issues through the slots. On top of the radiation surface, suitably fixed on the ring, is placed the load,

that is to say a pot or vessel with an absorber. Such a pot is shown in detail in FIG. 3. The pot is made of metal and is provided with a lid and a handle as well with a collar 23 in the lower portion which fits against the ring 19 when the pot is placed on the radiation surface. Through this the radiation is screened effectively from the surroundings. The base in the pot has two layers, a metal layer 24 and an absorption layer 25.-The metal layer may be very thin and serves on the one hand as a radiation protection in the pot and on the other hand as a roasting surface for the object in the pot. The absorption layer is formed by an active material, for example Fe O or NiFe O coal or a similar material, together with a binder and is placed directly on the metal layer. The two layers are therefore thermally very well attached and the energy taken up in the absorption layer is conducted with very small losses to the metal layer. The two layers have a low heat capacity and therefore heat up quickly when the absorber is exposed to radiation. On the other hand, the base of the vessel will cool down with appreciable delay when the radiation stops. Through this the advantage is obtained, that the cooking intensity, roasting temperature etc. in the pot react directly to a variation in the power supply, which considerably simplifies the setting of the power control when a specified intensity is required. Due to this radiation-protecting metal layer in the pot the same can be used without risk, even when the lid is removed.

The waveguide 12 present in the bottom of the oven together with mode converters is realized in principle similarly to those just described, except that a waveguide section 26 with a width approximately three times the width of the branch waveguide 12 is used. By means of the mode conversions, a portion of the TE wave will here be converted to both TE and TE waves. Thus the radiation surface 7 on this waveguide section can be made larger than the corresponding surface on the waveguide tube 15. The surface 7 is surrounded by a ring 27 and in this is fitted a round absorption plate similar to the one which forms the base of the pot according to FIG. 3. As the oven forms a closed space, no extra radiation protection is required, so that the metal layer on the absorption layer can be excluded on this absorption plate. As a result, the effect alluded to in the introduction is achieved, namely that an article on the plate is warmed up partly through contact pressure on the plate and, partly through microwave radiation which passes through the plate and has an effect on the article. Through the variation of the absorption capacity of the plate, for example with the help of the thickness, it is possible to alter the ratio of transmitted to absorbed radiation effect. A dish can thus be roasted on the surface by the heat which has been absorbed and is given off by the plate, at the same time as the microwave radiation warms the dish homogeneously right through. In this way a considerable amount of time is saved in the preparation without the dish being deprived of the desirable browned surface. Such a preparation which comprises both roasting and microwave heating may also be carried out in a pot such as that shown in FIG. 3, in which, however, the radiation-protecting metal layer 24 has been excluded.

In such a case, however, another radiation-protecting means must be arranged at the top of the pot, for example a lid, which is so equipped that it cannot be removed when the microwave power is switched on. The use of a pot for the preparation has naturally also the advantage that it can easily be removed and cleaned in the normal washing; this solves a considerable problem of hygiene in microwave ovens in general.

The principle of roasting and heating at the same time is in a wider sense also applicable to baking. The baking molds may, for example, be provided with an absorbing layer, which brings about that the mold remains warm so that a crust is formed on its contents, at the same time as a part of the radiation penetrates into the side of the molds and rapid-bakes" the contents.

A combination of oven and cooking plate according to the embodiment shown has the advantage that the scope of application for microwave heating is widened. Furthermore, the expensiveelectronic equipment can be used jointly for oven and plate; It is not to be regarded as essential, however, for the realization of the invention that this combination exists; a microwave heating appliance with an absorber may comprise only a cooking plate, only an oven, or either built into a conventional cooker.

The combination shown and described is thus only a preferred embodiment and should be regarded as one example of the application of the invention. This embodiment represents a basic form of the subject of the invention to which may be added further supplementary devices. Such devices consist, for example, of a temperature protection built into the absorber, which is coupled to a safety switch, which breaks the microwave power when the temperature on the absorber becomes too high. The invention in its entirety is defined in the following claims.

What is claimed is:

l. A microwave heating apparatus'including treatment means, a microwave source connected to a wave guide having a mode converter, said wave guide being provided with first and second sections, said first section having a width ofa and a height of b, said second section having a width equal to n'a' in which n 2,3,4, etc. and a height equal to b, said first section being connected to the microwave source, said second section being connected to said treatment means, said first section being joined to said second section by an- H-bend forming inside obtuse angles between the outer b sides of said first and second sections.

2. The apparatus as claimed in claim 1 in which one of said sides having dimension a of said first section is provided with an aerial inlet to which the microwave source is connected.

3. The apparatus as claimed in claim 1 in which the second section includes an applicator provided with slots through which microwave energy radiates.

4. The apparatus as claimed in claim 3 in which said applicator is arranged on a side dimensioned n'aand is defined by a number of slots therein.

5. The apparatus as claimed in claim 4 further including vessel means including a plate means having a high coefficient of microwave absorption converting microwave energy to thermal energy, and an external holder detachably fitting the vessel means.

6. The apparatus as claimed in claim 5 in which the plate means includes a layer of a high coefficient of microwave absorption.

7. The apparatus as claimed in claim 1 further including a branch section connected to said first section to extend at right angles thereto, said branch section having a width of a and a height of b, said branch section being connected to a further-section having a width approximately three times the width of the branch section, and a height of b, said further section being joined to said branch section by an l-l-bend forming inside obtuse angles between the outer b sides of said branch and said further section, and a switching means at the connection between the branch section and first section movable between a position oblique to the first section and a position parallel with the first section for directing radiation from said microwave source into the branch section and into the first section respectively. 

1. A microwave heating apparatus including treatment means, a microwave source connected to a wave guide having a mode converter, said wave guide being provided with first and second sections, said first section having a width of a and a height of b, said second section having a width equal to n.a. in which n 2,3,4, etc. and a height equal to b, said first section being connected to the microwave source, said second section being connected to said treatment means, said first section being joined to said second section by an H-bend forming inside obtuse angles between the outer b sides of said first and second sections.
 2. The apparatus as claimed in claim 1 in which one of said sides having dimension a of said first section is provided with an aerial inlet to which the microwave source is connected.
 3. The apparatus as claimed in claim 1 in which the second section includes an applicator provided with slots through which microwave energy radiates.
 4. The apparatus as claimed in claim 3 in which said applicator is arranged on a side dimensioned n.a. and is defined by a number of slots therein.
 5. The apparatus as claimed in claim 4 further including vessel means including a plate means having a high coefficient of microwave absorption converting microwave energy to thermal energy, and an external holder detachably fitting the vessel means.
 6. The apparatus as claimed in claim 5 in which the plate means includes a layer of a high coefficient of microwave absorption.
 7. The apparatus as claimed in claim 1 further including a branch section connected to said first section to extend at right angles thereto, said branch section having a width of a and a height of b, said branch section being connected to a further section having a width approximately three times the width of the branch section, and a height of b, said further section being joined to said branch section by an H-bend forming inside obtuse angles between the outer b sides of said branch and said further section, and a switching means at the connection between the branch section and first section movable between a position oblique to the first section and a position parallel with the first section for directing radiation from said microwave source into the branch section and into the first section respectively. 